We have identified a novel mammalian A-type cyclin, cyclin AI, that our targeted mutagenesis in mice revealed to be essential for the progression of male germ cells into meiosis. Human cyclin AI is also highly expressed in myeloid leukemia cell lines and in leukemic cells from patients with acute myeloid leukemias, in the promyelocytic form (APL) in particular. We have tested the hypothesis that the aberrant high levels of cyclin AI were causal in the leukemic phenotype, i.e., acting as an oncogene. Transgenic mice in which cyclin AI was expressed under the control of the human cathepsin G promoter in myeloid precursor cells were generated. They exhibited abnormal myelopoiesis and developed acute myeloid leukemia with low penetrance and long latency. Interestingly, in the transgenic mouse model and in human NB4 cells, the localization of cyclin A1 is predominantly cytoplasmic, distinct from its nuclear localization in germ cells. We wish to understand the cellular mechanisms in myelopoiesis that are altered in the presence of elevated levels of cyclin A1 that is now mostly cytoplasmic. The distinct cytoplasmic localization of cyclin A1 will be studied, testing the hypothesis that this property contributes to the tumorigenesis. We will also address the role of cyclin A1 during normal hematopoiesis by studying hematopoietic parameters in mice that are null for the cyclin A1 gene. The hypothesis that cyclin A1 will have distinct Cdk partners, other interacting partners, and substrates in normal versus leukemic cells will be tested using immunoprecipitation and a yeast 2-hybrid screen. As high levels of cyclin A1 protein have been shown to be characteristic of APL, we will ask whether manipulating the expression of cyclin A1 will affect the development of the leukemia. We will test this idea by performing genetic studies in which we will manipulate the expression of cyclin A1 in the fusion oncogene X-RARalpha transgenic animal models of APL. The question is whether these mice will be more resistant to the development of leukemia in the absence of cyclin AI. These studies will provide important insight into the etiology of myeloid leukemia, the role of cell cycle control in the oncogenic process, and the development of new and potentially highly tissue-specific target molecules for pharmacologic intervention.